Microwaves are not forms of heat but rather energy in the form of electromagnetic waves with frequencies that lie between 300 MHz and 300 GHz. Their wavelengths vary from 1 millimeter to 1 meter. The frequencies used for heating are those allocated by the Federal Communications Commission. These are the so-called ISM (industrial, scientific, and medical) frequencies, which have been set aside for applications in these specific areas.
The mechanism for applying microwave energy is quite different from that of conventional heating applications. The microwave system consists of a transmitter, waveguide, applicator, and controls as shown in the image above. Microwave systems are more compact, thus requiring a smaller equipment footprint. The transmitter may be located remotely in a dry, safe area. The waveguide may extend up to several hundred feet.
ECONOMICS – 915 MHz vs. 2450 MHz
A home microwave oven operates at 2450 MHz. Industrial/commercial microwave systems most commonly operate in the 915 MHz frequency, although some do use the higher 2450 MHz frequency. The lower frequency range used in industrial microwaves allows more efficient penetration of the microwave through the material. Typically there is about 3x’s greater wave penetration with 915 MHz than with 2450 MHz. With its higher total system efficiencies, 915 MHz heating and drying systems tend to have lower running costs than comparable 2450 MHz units. For example, IMS has found that one 100 kW 915 MHz transmitter will be about 50% cheaper to operate than seven 15 kW 2450 MHz units.
915 MHz units are suitable for many different applications as they can output up to 100 kW. Although the cost is similar, the largest commercial 2450 MHz units available use 30kW magnetrons. With their higher total system efficiencies, 915 MHz heaters and dryers tend to have lower running costs than comparable 2,450 MHz units. The 2450 MHz magnetrons are generally used for low volume R&D applications. The size of the magnetrons and wave-guides for a 2450 MHz system are considerably smaller than those used in 915 MHz units. This makes them suitable for small-scale installations.